Frequency modulation



`Iuxne 8, 1943. M, ARTZT 2,323,269

FREQUENCY MODULATION Filed Nov. '21, 1941 AAA VVVVVVV Ahum; vvvvvvv rwentor all".

Patented June s, 1943 UNITED STATES, PATENT oFFlc-Ey- FREQUENCY MODULATION Maurice Artzt, Haddoneld, 'N. J., assignor to Radio Corporation of America, a corporation oi' Delaware Application November 21, 1941, Serial No. 419,893

6 Claims.

My invention relates to frequency modulation circuits and particularly to an improved variable resistance modulating means for modulating an oscillator of the resistor-capacitor type.

An object of the invention is to provide animproved means for supplying a frequency modulated signal.

A further object of the invention is to provide an improved means for producing a. frequency shift of a carrier wave that is a high percentage of the carrier wave frequency.

A still further object of the invention is to provide an improved variable resistor network which will not introduce a variable voltage .into the oscillator or other associated circuit.

In one embodiment of my invention, there is employed an oscillator in which the output of a vacuum tube is coupled to its input circuit through a resistor-capacitor delay circuit. The delay network introduces a phase shift to cause oscillation at a certain frequency, this frequency being the one at which the phase shift in the network is 180 degrees. By varying the resistance of one or more resistors in the delay network, the frequency of oscillation may be changed. Thus, a vacuum vacuum tubes connected in series relation with` each other and connected across a, constant voltage source, the grid of one tube being connected to an intermediate point on this voltage source. The other vacuum tube functions as the desired variable resistor and, as will be explained, introduces no variable voltage.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figures l and 2 are circuit diagrams of two embodiments of the invention.

Figure 3 is a circuit diagram of the variable resistor network utilized in the circuits shown in Figures l and 2,

Figure 4 is a diagram showing the equivalent circuit for Figure 3, and

Figure 5 shows curves that illustrate the operation of the circuit as applied to facsimile, for example.

In the several gures like parts are indicated by the same reference characters.

In Fig. 1, the oscillator comprises a. vacuum tube I0 which may be a pentode having a cathode Il, a control grid I2, a screen grid i3, a suppressor grid I4, and an anode or plate I6.- The tube is self-biased by means of a cathode resistor Il bypassed by a condenser I8.

The output of the tube In is fea into a resistorcapacitor delay network comprising series capacitors lila, lab, Isc and I9d and shunt resistors 2i a, 2 Ib, 2 Ic, 2id and'2ie.- An operating voltagemay be supplied from a, B battery or other source (not c shown) to the anode I 6 through theshuntresistor Zia.

back through a conductor 22 to the control grid 2ic of the delay network is in series with a variable resistor in the form of a vacuum tube 23,` its associated tube 24 and an associated voltage di- The shunt resistance from the vider 26-21. junction point of condensers |917 and I9c to ground consists of the resistance of resistor 2Ic and the'plate-cathode impedance ofthe tube 23'. The tubes 23 and 24 may be triodes as shown'.v

each having a cathode, a control grid and an anode or plate.

plate-cathodeV impedance and thereby frequencymodulate the oscillator. for preventing the appearance of a varying voltage across the plate-cathode impedance of tube 23 during modulation are as follows:

The tubes 23 and 24 have their plate-cathode impedances connected in series, the plate of tube l 23 being connected to the cathode of tube 24. 'Ihe lower end of resistor 2|c is connected to the junction point of the plate and cathode so connected.

'I'he series combination of tubes 23 and 24 is connected across the voltage divider resistors 26 and 21, and the control grid of tube 24 is connected to an intermediate point, such as the midpoint, of the divider. Since the potential of the cathode of tube 24 will follow the potential of its associated control grid, this being cathode follower action, the cathode of tube 24 and the plate of tube 23 will remain at the constant potential' of the above-mentioned point on the voltage divider 26-2'i. Thus, the plate-to-cathode Delay voltage fromthe delay network is vfed'.-

In the example illustrated, the shunt resistor l Y 'Ihe modulating or control signal c is applied to the control grid of tube 23 to vary itsi The circuit connections voltage of tube 23l will be constant during modulation as desired. u l

The operation of the variable resistor circult may also be explained by referring to Fig. 3, where the variable resistor portion of Fig. l is shown with the parts slightly rearranged, and to the corresponding equivalent diagram in Fig. 4 where the plate-cathode impedances of the tubes 24 and 23 are represented by variable resistors RI and R2, respectively, and where resistor 2| c is indicated as Ro.

The impedances corresponding to resistors RI and R2 increase or decrease togeth'er with changes in signal voltage. For example-if the grid of tube 23 is made more negative, the tube impedance R2 increases, the resulting decrease in current through the serially connected tubes causes the cathode of tube 24 to become slightly more positive with respect to the grid of tube 24 wherebythe plate-cathode impedance RI of tube 24 also increases so that the potential at the variable resistance terminals indicated at Rv remains practically unchanged. Preferably, the potential at Rv is made zero by balancing the bridge formed by Rl, R2 and resistors 2B and 21. This may be accomplished, for example, by making resistors 2B and 21 equal and by employing tubes 23 and 24 that are alike.

The invention, which has many applications, has been found especially useful in facsimile transmission where the carrier wave at one frequency represents white and at a different frequency represents black." Such a carrier wave and its modulating or control signal are illustrated in Fig. 5. It will be noted that the carrier wave frequency changes almost instantly and substantially without the introduction of transients when the negative control pulse P is applied. This ls a very important advantage of the invention.

In Fig. 2 the circuit is similar to the one just described but it includes a cathode follower tube 3| for providing low impedance coupling into the delay network. Here the cathode resistor corresponds to the low impedance resistor 2Ia of Fig. 1.

Also, additional vacuum tubes 32 and 33 are included in the variable resistance network for the purpose of varying a second shunt resistor in the delay network, thus producing a greater shift in the oscillator frequency upon the application of a control signal. The plates and grids of tubes 24 and 32, and the cathodes and, grids of tubes 23 Network condensers [9a. etc mmf 470 Network resistors 2lb, 2id, 2Ie ohms 100,000l Network resistor 2IC do 30,000 Resistor Zia do 15,000 Resistors 26 and 21 ohms each-- 5,000 Plate-cathode impedance of tube 23 and of tube 24 ohms to infinity each-- 15,000

The invention may also be utilized to advantage in the transmission of speech or music since the modulation is linear within the modulation range of the circuit.

I claim as my invention:

l. An oscillator of the resistor-capacitor type which oscillates at a, frequency dependent uDOn the resistance ot an element in a resistor-capacitor circuit, said element comprising a variable resistance circuit which includes a pair of vacuum tubes having their plate-cathode `impedances connected in series with each other, each of said tubes having a control grid, said series combination of tubes being connected across a voltage source with the control grid of oneof said tubes connected to an intermediate point on said voltage source, means for connecting the platecathode impedance ol the other of said tubes into said resistor-capacitor circuit to function as at least a portion of said resistance element, and means for varying the plate-cathode impedance of said other tube in accordance with a control signal.

2. In combination, an oscillator of the type including a resistor-capacitor delay network for reversing the phase of the feedback voltage, and avariable resistance circuit included in said network for varying the frequency of the oscillator output, said variable resistance circuit comprising a pair of vacuum tubes having their platecathode impedances connected in series with each other, each of said tubes having a control grid, said series combination of tubes being connected across a voltage source with the control grid of one of said tubes connected to an intermediate point on said voltage source, means for connecting the plate-cathode impedance of the other of said tubes into said delay network to function as at least a portion of one of the resistors of said delay network, and means for varying the platecathode impedance of said other tube in accordance with a control signal.

3. In combination, an oscillator of the type including a resistor-capacitor delay network for reversing the phase of the feedback voltage, said network consisting of series capacitors and shunt resistors, and a variable resistance circuit included in said network for varying the frequency of the oscillator output, said variable resistance circuit comprising a pair of vacuum tubes having their plate-cathode impedances connected in series with each other, each of said tubes`having a control grid, said series combination of tubes being connected across a voltage source with the control grid of one of said tubes connected to an intermediate point on said source, means for connecting the plate-cathode impedance of the other of said tubes into said delay network to function as at least a portion of one of said shunt resistors in said network, and means for varying the platecathode impedance of said other tube in accordance with a control signal.

4. A ,variable resistance circuit which includes a pair of vacuum tubes having their plate-cathode impedances connected in series with each other and each having a control grid, said series combination of tubes being connected across a source of substantially constant voltage with the control grid of only one of said tubes'connected to an intermediate point on said voltage source, means for connecting the plate-cathode impedance of only the other of said tubes into a utilization circuit to function as a variable resistance element, and means for applying a control voltage to the grid of said other tube only for varying its plate-cathode impedance.

5, A Variable resistance circuit comprising a pair of vacuum tubes having their plate-cathode impedances connected in series with each other and each having a control grid, said series combination of tubes being connected across'a voltage divider with the control grid of only one of said tubes connected to an intermediate point on said divider, the bridge circuit formed by said tubes and said divider being balanced, means for connecting the plate-cathode impedance of the other of said tubes only into a utilization circuit, and means for applying a control potential to the grid of said other tube only for varying its platecathode impedance.

6. In combination, an oscillator of the type in.. cluding a resistor-capacitor delay network for reversing the phase of the feedback voltage, said network consisting of a plurality of recurring sections each consisting of a series capacitor and a shunt resistor, and a variable resistance circuit included in said network for varying the frequency of the oscillator output, said variable resistancecircuit comprising a pair of vacuum tubes having their plate-cathode impedances connected in series with each other. each of said tubes having a control grid, said series combination of tubes being connected across a voltage source with the control grid of one of said tubes connected to an intermediate point on said source, means for connecting the plate-cathode impedance of the other of said tubes into said delay network to function as at least a portion of one of said shunt resistors in said network.

means for applying a control signal to the control grid of said other tube for varying its plate-cathode impedance, and a second pair of tubes each having a control grid and also having their platecathode impedances connected in series with each other and across a voltage source with the control grid of one tube in said second pair connected to an intermediate point on said source, means for connecting the plate-cathode impedance of the other tube of said second pair into MAURICE ARTZT. 

